The present invention generally relates to rotary recording medium reproducing apparatuses, and more particularly to a reproducing apparatus which reproduces signals recorded on a rotary recording medium by varying the stylus pressure of a reproducing stylus according to the reproducing mode.
A new information signal recording and/or reproducing system has been proposed in U.S. Pat. Nos. 4,315,283, 4,322,836, and 4,331,976 of which the assignee is the same as that of the present application. According to this proposed system, the recording system forms pits in accordance with an information signal being recorded along a spiral track on a flat rotary recording medium (hereinafter simply referred to as a disc), without forming a groove therein. In the reproducing system, a reproducing stylus traces over along this track to reproduce the recorded information signal in response to variations in electrostatic capacitance.
In this system, since no grooves for guiding the reproducing stylus are provided on the disc, it becomes necessary to record pilot or reference signals on or in the vicinity of a track of a program information signal, on the disc. Upon reproduction, the reference signals are reproduced together with the program information signal. Tracking control is carried out so that the reproducing stylus accurately traces over along the track in response to the reproduced reference signals.
By use of this previously proposed system, there is no possibility whatsoever of the reproducing or the disc being damaged since the recording track has no groove. The reproducing stylus can trace over the same portion of the track repeatedly many times, whereby a special reproduction such as still picture reproduction, slow-motion reproduction, and quick-motion reproduction becomes possible in a case where the recorded information signal is a video signal.
In order to carry out the above special reproduction, a kick pulse is produced by a special reproducing mode control circuit, and this kick pulse is applied to a tracking control coil in a signal pickup reproducing device, to shift the reproducing stylus to an adjacent track. Because the track is formed in a spiral manner, upon still picture reproduction, for example, the reproducing stylus is shifted to an adjacent track in the outer peripheral direction of the disc, once for every revolution of the disc. During the slow-motion reproduction, the reproducing stylus is shifted to an adjacent track in the outer peripheral direction of the disc, once for every predetermined number of revolutions over one inclusive of the disc. Further, upon quick-motion reproduction, the reproducing stylus is shifted to a track in the inner peripheral direction of the disc.
When manufacturing the disc, recording is first carried out with respect to an original disc which is coated with a photoresist, by using a laser beam which is light-modulated by the recording signal. Then, this original disc is subjected to processes such as developing process, plating process, and the like, to obtain a master disc. A mother disc and a stamper are successively obtained from this master disc. This stamper is used for molding a synthetic resin which is admixed with conductive particles, so as to form the disc. The disc thus formed is formed with pits in accordance with the recording signal When a reproducing stylus provided with an electrode traces over this disc, the electrostatic capacitance between the disc and the electrode of the reproducing stylus varies according to the pits formed on the disc, and the recorded signal is reproduced according to this variation in electrostatic capacitance.
However, if extraneous particles exist on the original disc or the stamper during the above manufacturing process, concave portions corresponding to the extraneous particles will be formed in the stamper, and a disc formed by such stamper will be formed with convex portions. In addition, when foreign substances are mixed into the molding material upon forming of the disc and the stamper becomes scratched, a disc formed by such stamper will be formed with convex portions in correspondence with concave portions of the scratches on the stamper.
The length or width of the undesirable convex portion formed on the surface of the disc is in the range of several tens of .mu.m to several hundred .mu.m, and the height is in the range of several .mu.m to several tens of .mu.m, for example. On the other hand, the track pitch of the pits of the recorded signal is 1.35 .mu.m, and the depth is 0.2 .mu.m to 0.4 .mu.m, for example. The disc is rotated at a rotational speed of 900 rpm, for example. If the stylus pressure of the reproducing stylus which traces over the surface of the rotating disc is small, there will be a phenomenon in which the reproducing stylus will separate from the disc surface due to the shock introduced when the reproducing stylus hits the convex portion formed on the disc surface and then make contact with the disc surface again at a part of the disc well past that convex portion. While the reproducing stylus is separated from the disc surface due to the hitting of the convex portion formed on the disc surface, the recorded signal will not be reproduced, and signal dropout will be introduced.
Normally, a known dropout compensation circuit is provided within the reproducing circuit. When signal dropout occurs, the dropout compensation circuit carries out the dropout compensation by use of a signal which is obtained by delaying a signal within a horizontal scanning period prior to the horizontal scanning period in which the dropout exists. The signal dropout introduced due to the convex portion having the above described size formed on the disc surface, can be compensated at the dropout compensation circuit. However, the signal dropout introduced due to the phenomenon in which the reproducing stylus jumps and separates from the disc surface when hitting the convex portion formed on the disc surface, has a long duration timewise, and it is not always possible to compensate for this kind of signal dropout by the above dropout compensation circuit.
The main body of the reproducing stylus is normally formed of a hard material such as diamond. According to experiments performed by the present inventor, it was found that when the stylus pressure of the reproducing stylus is set large, the reproducing stylus may scrape off the convex portion formed on the disc surface. However, if the stylus pressure of the reproducing stylus is constantly set large so as to scrape off the convex portion formed on the disc surface, the wear of the reproducing stylus and the disc become large during the still picture reproduction in which the same track is repeatedly traced, for example, and there will be a problem in that the serviceable lives of the reproducing stylus and the disc will become short.
On the other hand, if a quick-motion reproduction is to be carried out by successively shifting the reproducing stylus to adjacent tracks, a large stylus pressure will be needed because the whole reproducing stylus must successively scrape off the convex portions formed on the disc surface.